Separators for use in electrochemical devices, in particular in secondary batteries, are needed to keep the anode physically and electrically separated from the cathode of the electrochemical cell, while permitting flow of electrolyte ions.
Separators must be chemically and electrochemically stable towards the electrolyte and the electrode materials and must be mechanically strong enough to withstand high tensions generated during battery assembly operations.
Further, their structure and properties may considerably affect battery performances, including energy density, power density, cycle life as well as safety.
For high energy and power densities, the separator is required to be very thin and highly ionically conductive, while still remaining mechanically strong.
During battery operation, thermal runaway may occur, which may cause dimensional shrinking or melting of the separator, eventually leading to physical contact of the electrodes and to internal short circuit and fire hazard.
For battery safety, the separator must be able to shut the battery down when overheating occurs, so as to avoid the thermal runaway.
Inorganic composite membranes have been widely used as separators for electrochemical devices including secondary batteries, in particular Lithium-ion batteries. Examples of composite porous films for use in membranes are disclosed in. US 2002/197413 A (TEIJIN LTD) 13 Sep. 2001
WO 2015/022229 (SOLVAY S.A.) discloses a process for manufacturing a solid composite separator comprising coating a porous substrate with a composition including a copolymer of VDF/HFP/HEA and silica, followed by a drying step and then, optionally by a curing and by a compression step.
US 2014/315080 (ABUSLEME) discloses a hybrid VDF-HFP-HEA/silica polymer obtained by reaction of TEOS (tetraethylorthosilicate) with a composition including a copolymer VDF-HFP-HEA.
Also, a low thickness of the separator is required for high energy and power densities. However, this adversely affects the mechanical strength of the separator and the safety of the battery thereby provided.
A variety of inorganic filler materials have been long used to fabricate inorganic composite membranes wherein inorganic particles are distributed throughout a polymeric binder matrix.
Although inorganic composite membranes offer excellent wettability by the electrolytes, good thermal stability and zero-dimensional shrinkage at high temperatures, they are usually not mechanically strong enough to withstand handling in cell winding and assembly.
In particular, separators used in wound electrochemical cells require a high mix penetration strength to avoid penetration of electrode materials through the separator. If particulate materials from the electrodes penetrate the separator, a short circuit will result.
In many cases, the inorganic composite membrane contains a very high content of inorganic filler materials. In some instances, the so-obtained inorganic composite membrane exhibits poor mechanical strength and tends to break down during handling, hence they ultimately result very difficult to use in the manufacture of assemblies for electrochemical cells.
One particular challenge has been thus to provide for composite membranes with acceptable thickness to be suitably used as separators in electrochemical devices.
Multilayer composite membranes can be obtained using multiple coating steps. However, multiple steps disadvantageously increase processing costs.
There is thus still the need in the art for an solid composite separator having high ionic conductivity, to be suitably used in electrochemical devices, that exhibits also outstanding thermal and mechanical properties necessary during assembly and/or operation of the same devices, and for the process for its preparation.